Sift together flour, baking soda, and salt; set aside.
Using an electric mixer, cream together butter, peanut butter, 1/2 cup granulated sugar, and light brown sugar.
Add egg, milk, and vanilla; beat until well blended. Gradually add the flour mixture, mixing thoroughly. If the dough is very soft, refrigerate for about 1 hour.
Preheat oven to 375 degrees. Spray, oil, or line a cookie a sheet with nonstick liner and set aside.
Roll dough into 1-inch ball. Roll cookies in sugar and place 2 inches apart on the cookie sheet.
Bake until very light brown and puffed, 6 to 8 minutes. Remove the sheet from the oven and lightly press a candy kiss into the center of each cookie, allowing it to crack slightly.
Return to oven until light golden brown, 2 to 3 minutes. Remove from oven, cool completely, and store in an airtight container.